1. Technical Field of the Invention
The present invention relates to a heating device and a heating method applicable to: a fixing device provided in a dry electrophotographic apparatus; a drying device provided in a wet electrophotographic apparatus; a drying device provided in an inkjet image forming apparatus; an erasing device for rewritable media, etc.
2. Description of Related Art
While there are several types of heating devices applicable to a fixing device provided in an electro-photographic image forming apparatus, a drying device provided in an inkjet image forming apparatus, and the like, an image forming apparatus of internal heating type is widely in use. Generally, the image forming apparatus of internal heating type is provided with a heat supplier as a heat source such as a halogen heater, inside a heating member such as a heat roller composed of a hollow metal core made of aluminum or the like. The surface of the heating member is heated by the heat supplier to be set at a predetermined temperature, thereby heating a heated member such as a paper sheet.
However, the image forming apparatus of internal heating type has had the following drawbacks. In the image forming apparatus of internal heating type, it takes a long time before the surface of the heating member reaches a predetermined set temperature after the heating member starts to be heated. Namely, what is called warm-up time is long. Accordingly the heating member needs to be maintained at a certain temperature even during a standby period, for example for quick recovery from a standby state to a normal operation state. Consequently, the heating device has high standby power consumption.
As a heating device which enables a shorter warm-up time and lower standby power consumption, the Japanese Publication for Laid-Open Patent Application No. 10-133505 has disclosed a heating device of external heating type wherein the surface of a heating member is heated by a heat supply roller arranged in the vicinity of the heating member in such a manner as to be in contact with the surface of the heating member.
Since the surface of the heating member is in contact with a heat source to supply heat thereto, the heating device of external heating type has high thermal efficiency, thereby allowing a warm-up time to be reduced greatly.
Compared with the heating device of internal heating type, however, a conventional heating device of external heating type has had the two following drawbacks, which is the case with the one as disclosed in the Japanese Publication for Laid-Open patent Application No. 10-133505.
First, the heating device of external heating type has low heat-supply performance in heat supply to the heating member.
FIG. 1 illustrates results of calculating from a two-dimensional heat transfer analysis a ratio of heat energy used to heat a heat roller as a heating member to electrical energy supplied to a halogen heater as a heat source, in respective heating devices of internal and external heating types. The main conditions of the above calculation are: diameter of heat roller: 40 mm, both in internal and external heating types; diameter of heat supply roller as a first heat supplier: 15 mm; temperature of heat supply roller: 200° C.; rated apparent power: 1200 W.
In the heating device of external heating type, since the heat supply roller as the heat supplier is arranged outside the heat roller, heat energy is lost by the amount of heat escaping from the heat supply roller into the atmosphere. Therefore, heat transfer efficiency in heat transfer from the heat source to the heat roller is lower than in the heating device of internal heating type.
Further, in the heating device of external heating type, there is a limit to a surface temperature of the heat supply roller depending on heat-resistant temperatures of a material thereof and a material used for the surface of the heat roller. If a fluorocarbon resin material is in use for the surface of the heat roller, for example, the allowable maximum temperature of the surface of the heat supply roller is approximately 200° C. Consequently, electric power supplied to the heat supply roller for supplying heat to the heat roller is also limited, with the result that transferred heat energy to the heat roller becomes approximately one-quarter as high as transferred heat energy in the heating device of internal heating type.
Second, the heating device of external heating type has high standby power consumption. More specifically, while enabling much shorter warm-up time compared to the heating device of internal heating type, the heating device of external heating type requires warm-up time of approximately 30 seconds, and thus still needs to be preheated during a standby period.
For preheating the heat roller during the standby period, accordingly, the heat supply roller and the heat roller must be driven to rotate with both of the rollers pressed against each other. This leads not only to higher standby power consumption by a drive system or the like of each of the rollers, but also to shorter life of the heat roller.